The present invention relates to new semiconducting organic compounds and/or polymers for use in optoelectronic devices that contain completely or partially deuterated conjugated backbones, which can promote luminescence and thermal stability of the materials. More particularly, the present invention relates to the effective modification of the molecular structure of known luminescent materials with conjugated backbones containing hydrogen atoms by replacing one or more of the hydrogen atoms with deuterium atoms. The resulting deuterated material is significantly altered and has greatly improved performance over known luminescent light-emitting materials.
Organic semiconductors have the benefit of low cost processing, easy control of properties by changing chemical structures, and attractive electronic properties. These materials are usually composed of conjugated chromophores linked by saturated or unsaturated linkage units. Many interesting applications have been explored in recent years, such as organic light-emitting devices [C. W. Tang, S. A. Van Slyke, Appl. Phys. Lett., 1987, 51(12), 913; J. H. Burroughes; D. D. C. Bradley; A. R. Brown; R. N. Marks; K. Mackay; R. H. Friend; P. L. Burn and A. B. Holmes, Nature 1990, 347, 539. R. H. Friend; R. W. Gymer; A. B. Holmes; J. H. Burroughes; R. N. Marks; C. Taliani; D. D. C. Bradley; D. A. Dossantos; J. L. Bredas; M. Logdlund and W. R. Salaneck, Nature 1999, 397, 121.], organic lasers [F. Hide; M. A. Diaz-Garcia; B. J. Schwartz and A. J. Heeger, Acc. Chem. Res. 1997, 30, 430.], organic thin-film transistors [F. Garnier, R. Hajlaoui, A. Yassar, P. Srivastava, Science, 1994, 265, 1684], and organic photodiodes.
One application of organic semiconductors is as an active flat-panel display or organic light-emitting device (OLED). Such devices offer many unique features, including a low cost, full color, active display with the possibility of a thinner, lighter, larger and more flexible display module, with wider viewing angles ( greater than 160xc2x0), compared to conventional flat-panel display devices. All these features imply a strong competitive alternative to replace present LCD displays. Such devices consist of one or several semiconducting organic layer(s) sandwiched between two electrodes. When an electric field is applied, electrons are injected by the cathode into the lowest un-occupied molecular orbital (LUMO) of the adjacent molecules, and holes are injected by the anode into the highest occupied molecular orbital (HOMO). As a result of recombination of electrons and holes, an excited state, called singlet exciton, is formed which returns back to the ground state upon emission of light corresponding to the energy band gap of the emissive material. The selection of emissive materials not only can influence the emission color, but also the light emission efficiency (photons per injected electron) and the light emission brightness and lifetime. In practical display applications, color purity, light-emission efficiency, brightness and lifetime are important parameters.
There have been various organic semiconductor materials developed in the past a few years [Y. Sato, Semiconductors and Semimetals, 2000, 64, 209; A. Kraft; A. C. Grimsdale and A. B. Holmes, Angew. Chem.xe2x80x94Int. Ed. In Engl. 1998, 37, 402; Li, X.-C., Moratti, S. C.; In Photonic Polymer Systems: Fundamentals, Methods, and Applications; Wise, D. L.; Wnek, G. E.; Trantolo, D. J.; Cooper, T. M.; Gresser, J. D.; Eds.; Marcel Dekker, Inc.: New York, Chapter 10, 1998, p. 335]. Many prototypes of OLED display modules have been demonstrated with the use of organic luminescent compounds (small molecular compounds or polymers). However, only limited commercial products of OLEDs have been launched, due both to the difficulty of technological integration and to the overall performance of the present organic semiconducting materials, which include emissive materials and charge transporting materials. Accordingly, there is a need in the art to develop new materials that exhibit combinatory high performance of luminescence, excellent stability, and good lifetime.
The search for new organic materials used for optoelectronic devices, such as organic light-emitting devices, has been a very active field in recent years. This includes research relating to organic semiconductors and organic polymeric materials that present strong luminescence and good processibility. Most researchers have focused on varying the structure of either the core chromophore and/or polymer backbone, and on modifying linkages for solubility, for charge injection ability, or for other processing functionality. These methods require innovative molecular design, coupled with skillful chemical synthesis, which are time consuming and expensive in order to screen off undesired products.
It is an object of the present invention to provide a unique and effective manner to chemically modify known and novel optoelectronic materials by replacing protons with deuterium atoms on the conjugated chromophores. It is another object of the invention to provide a method to design and synthesize new luminescent organic materials containing deuterium atoms for their application in optoelectronic devices, such as light-emitting devices. It is further another object of the invention to improve OLED performance with brighter luminance and better thermal stability.
The present invention relates to deuterated semiconductor organic compounds used in optoelectronic devices and to processes of preparing such deuterated compounds. In one process within the scope of the present invention, known and novel organic semiconductor compounds are deuterated by replacing one or more hydrogen atoms covalently bonded to carbon atoms with deuterium atoms.
Deuterium is a non-radiative isotope of hydrogen, which is sometimes called heavy hydrogen due to its doubled atomic mass. The difference between hydrogen and deuterium has fairly small chemical effects; however, there are important physical effects because of the mass difference between the isotopes. The heavier isotope, deuterium, lies lower in the potential well, and hence has a lower zero-point energy and vibration frequency, and smaller vibration amplitude than hydrogen. Because of the asymmetry of the potential well, bond lengths and bond angles involving deuterium are different than those involving hydrogen. The observed smaller amplitude of the C-D stretching and bending motion, relative to C-H, should be best accounted for with a smaller van der Waals radius for D than for H. The weak vibronic coupling in the deuterated system has been used to theoretically predict higher fluorescence quantum yield [A. L. Burin and M. A. Ratner, J. Chem. Phys., 1998, 109, 6092]. Spectroscopic studies indicate that deuterated phenanthrene has a smaller non-radiative triplet rate constant than its aromatic molecule [S. M. Ramasamy, R. J. Hurtubise, Appl. Spectroscopy, 1996, 50(9), 1140].
Deuterium is also found to act as an apparent electron-donating inductive substituent relative to hydrogen. The isotope effects may be applied in the design of new luminescent materials with enhanced charge-injection ability. Additionally, it is know that the C-D bond is shorter than the C-H as a result of the anharmonicity of the bond stretching potential. [M. L. Allinger and H. L. Flanagan, J. Computational Chem. 1983, 4(3), 399]. This means the carbon-deuterium chemical bond is stronger, more stable, and reacts more slowly than the carbon-hydrogen chemical bond, so that the deuterated organic system has better thermal stability, and longer lifetime in optoelectronic devices. Deuterated luminescent material may also have a higher electroluminescent quantum yield as a result of smaller non-radiative triplet rate.
In the prior art, deuterated hydrocarbon lubricants have better anti-oxidation and improved stability than normal hydrocarbon lubricants [U.S. Pat. No. 4,134,843]. A deuterated polymer has been used for optical fiber with low attenuation optical loss [U.S. Pat. No. RE031,868]. Deuterated pharmaceuticals or drugs can enhance drug""s efficacy and activity. [U.S. Pat. No. 4,898,855 and U.S. Pat. No. 5,846,514]. Deuterium-treated semiconductor devices have been disclosed [U.S. Pat. No. 5,872,387], wherein degradation of inorganic semiconductor devices has been reduced by using deuterium passivation; deuterium incorporation at the SiO2/Si interface has been reported to improve the hot carrier reliability of CMOS transistors. [J. Lee, K. Cheng, et al, IEEE Electron Device Letters, 2000, 21(5), 221]. Compared with normal hydrogen treated device, the deuterium treated device has a significant lifetime improvement (90 times).
The deuterated organic semiconductor materials within the scope of the present invention are preferably luminescent. Luminescence here means either fluorescence (singlet emission) or phosphorescence (triplet emission). The deuterated organic semiconductor materials may possess charge injection (electron injection or hole injection), hole blocking, or exciton blocking properties. As used herein, hole blocking property means that the semiconductor material allows electrons to transport, but not holes. Whenever a material has a low mobility for hole transporting (less than 10xe2x88x926 cm/Vxc2x7s), or a very high HOMO (highest occupied molecular orbital) level, the material normally possesses hole blocking properties. As used herein, exciton blocking property means that excitons are confined within an emissive layer by using another material layer which does not readily transport excitons (usually non-emissive layer).
The deuterated organic semiconducting material preferably possesses a strong energy transfer property. As used herein, energy transfer includes a Forster process where a higher energy singlet transfers to a lower energy singlet. An example is a blue emissive polymer (host) doped with red emissive material (guest) [like tetraphenylporphyrin doped polyfluorene reported by T. Virgili, et al., Adv. Mater., 2000, 12(1), 58]. The doping level is usually from 0.1-15%, but more than 90% energy will transfer into the red emissive material, thus lead to red emission rather than blue emission. As used herein, energy transfer also includes an intersystem transfer where the singlet energy transfers to a triplet, and thus lead to phosphorescence. An example is a yellow emissive polymer doped with rare earth metal complexes to lead to electrophosphorescence [M. D. McGehee, et al., Adv. Mater., 1999, 11(16), 1349; Appl. Phys. Lett., 1999, 75, 4]. This triplet-emission can potentially lead to very high quantum efficiency because the maximal probability of a triplet is 75%.
The deuterated organic semiconducting materials preferably produce more singlet energy states for light-emission, compared to non-deuterated materials. With the injection of holes and electrons into a luminescent organic semiconductor or a luminescent polymer, an excited state, called exciton, is formed. For most organic fluorescent materials, the maximum singlet exciton production probability under electrical excitation is about 25%. Deuterated materials may exceed that limit for electrofluorescence because of their slow triplet production rate. Therefore, deuterated materials may efficiently produce more singlet excitons than non-deuterated materials, with a potential singlet exciton production exceeding 25%. Since the EL light emission efficiency is directly related with the production rate of singlet exciton, higher exciton production results in higher EL efficiency.